A variable gain amplifier is an electronic amplifier that varies its gain depending on a control voltage (often abbreviated CV). The variable gain amplifier can be fabricated as a CMOS or BiCMOS circuit and has many applications, including handling dynamic signal level in transmitter and receiver of a radio device for radio-freq and microwave applications. For example, variable gain amplifiers have been widely used as an AGC circuit (automatic gain control) to control gains in order to obtain a fixed output signal level for a fluctuating modulated input signal level. The reliability of demodulation is increased by applying a fixed signal level to the demodulator at the later stage of processing, and high signal-to-noise ratio (S/N ratio), i.e. small signal distortion, also increases the reliability of demodulation.
More particularly, when an input signal level to the variable gain amplifier is small, an AGC circuit having low noise characteristics is required at receiver to obtain the high signal-to-noise ratio described above. It is demanding at radio that a high linearity (low inter-modulation), which often characterized as IP3, can be obtained across input signal range.
In general, in order to have high linearity (low inter-modulation), an amplifier having a negative feedback structure is used to increase input level linearity. However, in known circuits, there is poor input impedance matching at differential AGC (automatic gain control) levels due to de-generation, as well as low dynamic range and no high IP3 at high gain.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.